1. Field of the Invention
The present invention relates to a quantum cascade laser.
2. Related Background Art
A structure of a Quantum Cascade Laser (QCL) is disclosed, for example, in Patent Literature 1 (U.S. Pat. No. 5,457,709). The operation of this quantum cascade laser will be described with reference to FIG. 4. FIG. 4 shows the band structure of a core layer in the conventional quantum cascade laser. As shown in FIG. 4, the core layer of the quantum cascade laser has the structure in which unit structures each consisting of one active layer and one injection layer of a quantum well structure are repeatedly formed in several ten cycles. An electric field is applied to this quantum cascade laser in a direction D11 indicated by an arrow in FIG. 4, to inject electrons into the active layer by resonant tunneling. The injected electrons transit from an upper energy level to a lower energy level of the active layer, whereby the active layer emits light of an emission wavelength corresponding to an energy difference between the upper energy level and the lower energy level, according to the transition. After the transition to the lower energy level with the emission of light, the electrons are quickly relaxed from the lower energy level to the ground energy level (relaxation energy level) by LO phonon scattering to migrate into the injection layer.
The behavior of electrons as described above is caused by such design that the energy difference between the lower energy level and the ground energy level is set close to the energy of LO phonon so as to resonantly induce LO phonon scattering. When the LO phonon scattering is resonantly induced, the electrons at the lower energy level are relaxed to the ground energy level in a relatively short scattering time. This relatively quick LO phonon relaxation process causes a population inversion between the upper energy level and the lower energy level. Since electrons migrating from the ground energy level into the injection layer are designed to be injected into the active layer of the next unit structure, the process as described above is repeated in several ten cycles by the number of unit structures, so as to achieve a large gain eventually, thereby enabling lasing. In this manner, the quantum cascade laser performs the laser operation of the three energy levels (upper energy level, lower energy level, and ground energy level) in the conduction bands of the quantum well structures.
Conventionally, active layers composed of GaAs and barrier layers composed of AlGaAs are grown on a GaAs substrate by an epitaxial growth method such as MBE and MOCVD, for example. And active layers composed of GaInAs and barrier layers composed of AlInAs or AlAsSb are also grown on an InP substrate by MBE or MOCVD.